What Is The Pollen Count, And What Affects It?

What is pollen?

Essentially pollen particles are the male cells of plants, which need to find the stigma, female cells, of a plant for pollination to be successful.

Not all pollen is bad!

A lot of plants with either self-pollinate, or use carriers such as bees, butterflies and other insects. It’s the plants that release their male cells (pollen) into the air for cross-pollination that cause the problem, because they are easily inhaled by humans, cats and dogs (yes, your pets can get hay fever too!).

How is pollen measured?

Through monitoring sites across the UK, pollen is physically collected using a Burkard trap.

The placement of these traps is well thought out – too low and the sample air will be tainted with too many pollutants and bits of dust, too high, and they’ll only collect tree pollen. So, they’re generally placed on roofs to sample air that is an even mix.

Inside the trap is a spindle with sticky paper wrapped around it. This rotates slowly and air is drawn through a slit in the casing surrounding the spindle. As air flows through the trap pollen particles are left on the paper ready for counting using a microscope.

This then gives us the amount of pollen per cubic metre observed over 24 hours, resulting in what we know as the ‘pollen count’.

The ‘pollen forecast’ is predicted using both the counts and the weather forecast to calculate the predicted risk of pollen levels over the coming days.

What is a ‘high’ pollen count?

This is worked out on the level of pollen per cubic metre that causes the average person to experience hay fever symptoms. Usually, this is when the count exceeds 50, although every individual is different, and will have different tolerance levels.

The pollen forecast is usually given as a Low, Moderate, High, or Very High levels.

What’s regarded as ‘high’ will change depending on the type of pollen:

• Trees – high = 208 - 703 grains of pollen in every cubic metre of air
• Grass – high = 50 to 349 grains of pollen in every cubic metre of air
• Weed – high = 78 to 266 grains of pollen in every cubic metre of air

*MET Office

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How the weather affects pollen count

Rain, wind, temperature and sunshine all play a significant role in the levels of production, distribution and dispersion of pollen.

Rainfall and pollen

Any rainfall will cause a decrease in pollen concentration, but it’s not as simple as that! The time and amount that it rains during the day will also have an influence.

Early, heavy and prolonged rain is likely to keep counts low all day whereas rain in the afternoon will have less of an impact.

If it rains around a thunderstorm, then the humidity in the air can cause pollen grains to burst open, releasing a high density of pollen into the surrounding air.

Time of day

Whilst pollen release depends on the species of plant, causing symptoms to peak at different times, in general pollen levels will have two peaks, early morning and late evening.

There is an initial release and lift in the first part of the morning, with levels remaining high until midday/early afternoon. Then another peak occurs when settling takes place at the end of the day. This settling can take longer in urban areas due to the extra heat, so pollen levels can remain high during the night.

Temperature and pollen

Temperature plays an important part in the release of pollen. For grass, a maximum temperature between 18 - 28 °C could give a high count if it’s a dry day with low humidity and a gentle breeze.

Trees respond best when the temperature range is between 13 - 15 °C. However, if the temperature rises above 28 °C then all pollen levels decrease. If several warm days occur in a row, then the supply of pollen can run out altogether.

Wind and pollen

When it comes to wind then it all becomes a little more complicated. If the wind is too light the pollen will barely get off the ground to be dispersed, but if it is too windy, the pollen gets blown further afield and thins out significantly.

Different types of pollen will need different wind speeds for ideal dispersion depending on the size, for example some tree pollen is larger and needs moderate to high winds, whereas smaller pollen from weeds only needs a breeze.

Sunlight and pollen

The amount of daylight, or the “photoperiod”, is also crucial to pollen production because of photosynthesis. If there is a particularly cloudy spell of weather, then plants and trees will produce less pollen because they are getting less light.

In summary, the 'worst' weather conditions for hay fever sufferers are a warm, dry and windy day, and the 'ideal' conditions are a rainy, low humidity, windless day.

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